Using optical fiber cable as a distributed acoustic sensing cable is an emerging technology, potentially serving as a next-generation downhole acoustic/seismic logging tool for vertical seismic profiling. The principle is to use optical fiber to listen to acoustic events from a surrounding medium. A fiber optic cable is typically deployed from a wellhead into a borehole, and every section of the cable length acts a hydrophone or geophone. The optical fiber becomes attractive because it enables vibration sensing, similar to a single component of a geophone, when every meter of a fiber sensing cable can detect vibration at its location separately from all other locations, allowing for thousands of vibration sensors on a single cable. From the detected acoustic signals, one may easily perform a vertical seismic profile of a borehole.
For a wireline deployment of the sensing cable, where the cable is not clamped to tubing or cemented in behind the wellbore casing, the observed borehole stratigraphy is often contaminated by various noises specifically from acoustic sensing cable-borne waves, excited by surface vibration near the wellhead and many other artificial sources. In this case, any mechanical movement of the wellhead, excited by machine vibration and natural events, could induce downward acoustic cable waves (longitudinal and shear) propagation along the cable. On the other hand, these cable waves may be created by downhole fluid (liquid or mud) flow, specifically at casing joints or a dramatic change in borehole diameter.
Existing distributed acoustic sensing cables are based on optical fibers sealed within hollow metal tubing. The field installation is to feed the cable through the wellhead and freely hang the cable inside the wellbore with a weight or a clamping tool at the end of the cable. When a fiber optic cable is deployed from the wellhead into the borehole, the detected acoustic signals may be easily contaminated by various acoustic noises. Acoustic sensing cable waves are excited by the wellhead vibration, for example, and can propagate long distances without significant decay, and can be reflected at different acoustic impedance interfaces. Field trials have shown that the recorded vertical seismic profiles with distributed acoustic sensing technology deployed via wireline or slickline often contain high-amplitude cable waves that greatly degrade the observed borehole seismic data quality. In fact, acoustic cable waves can propagate a long distance without significant decay and can be reflected at different acoustic impedance interfaces multiple times. These acoustic waves are guided along the cable and remain robust over a long propagating distance. In a more complicated case, downward and upward propagated cable waves from multiple reflections may induce complicated effects that cannot be fully removed from the recorded seismic data from subsequent processing steps. Previous efforts in removing such cable-borne noise included changing the tension of the sensing cable below the wellhead assembly. However, this cable slack may potentially cause sensing cable warping. Another effort included modeling and signal processing with different filters, such as velocity filter, low- and high-filters, etc. Post-data processing with different filtering methods may eliminate some of the noises but no effective method has been found to be capable of fully removing the cable-wave induced acoustic noises.